1. Field of the Invention
The present invention relates to the field of piston engines.
2. Prior Art
Historically, piston engines have used mechanically actuated poppet type intake and exhaust valves operated by way of an engine driven camshaft. While such systems are in a high state of development and usually provide reliable performance for the life of the engine, they have the disadvantage of providing a fixed relationship between crankshaft angle and valve position. Accordingly, the timing for valve opening and closing, the valve lift obtained, etc., are predetermined and fixed throughout the operating range of the engine, thus providing a substantial engine performance compromise under most engine operating conditions.
More recently, considerable work has been done in the development of alternate engine valve actuation systems, generally with a goal of allowing the varying of valve opening and closing crankshaft angle with varying engine operating conditions, and in some cases, of varying the valve lift based on engine operating conditions. One such alternate actuation system comprises hydraulic valve actuation using a spring return, a hydraulic return, or a combination of both. Generally, such valve actuation systems use either a single stage or a two-stage electrically controlled valving system for operation of the hydraulic actuator, the valving system being operative between three states, the first coupling the hydraulic actuator to a source of hydraulic fluid under pressure, the second blocking hydraulic fluid communication to or from the hydraulic engine valve actuator, and the third coupling the hydraulic engine valve actuator to a low pressure drain or vent. Thus engine valve lift may be controlled by controlling the timing between initiating valve opening by coupling the hydraulic engine valve actuator to the source of fluid under pressure and the blocking of the flow of hydraulic fluid to or from the hydraulic engine valve actuator. This, in theory, provides the desired result, though in practice may not provide the accuracy and uniformity in valve lift desired for smooth engine operation under all conditions.
Systems are also known for controlling the valving based on actual measurement of valve position. This has certain advantages, but also adds to the complexity of the system. In engines with multiple intake and/or exhaust valves per cylinder, a common engine valve actuator for the multiple valves of each type would need to be used, as typically the control would be common for economic reasons, and separate actuators may not track each other that well. Also, the control would need to be closed loop in real time for each actuator, preferably with a self adapting capability based on feedback of the actual lift obtained on the last engine cycle, making the control algorithm complicated and limiting the accuracy achieved by the limited speed of the control valving.